Chargaff's rule states that DNA from any species of any organism should have a 1:1 stoichiometric ratio (base pair rule) of purine and pyrimidine bases (i.e., A+G=T+C) and, more specifically, that the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine. This pattern is found in both strands of the DNA.

Calculation:

Since you know there are only four bases in DNA, all four bases together must equal 100 percent of the sample. If given the information that the sample is 20 percent guanine, you can surmise it is also 20 percent cytosine since guanine and cytosine pair with each other. Together, that is 40 percent of the total sample. You can subtract that 40 percent from 100 percent and determine 60 percent of the sample must be adenine and thymine together. Since those two bases always exist in equal concentrations, you know the DNA sample is 30 percent adenine: